restore: use /proc/self/mountinfo for collecting mounts fo the root task (v3)

If the root task is forked in a new pidns, it can't use its pid for
accessing /proc, because this proc belongs to the source pidns.

v2: don't copy a static string.
v3: take a bright part of Tycho's patch
Reported-by: Tycho Andersen <tycho.andersen@canonical.com>
Cc: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

mount.c

@@ -1911,7 +1911,7 @@ int prepare_mnt_ns(void)
 {
 	int ret = -1;
 	struct mount_info *mis, *old;
-	struct ns_id ns = { .pid = getpid(), .nd = &mnt_ns_desc };
+	struct ns_id ns = { .pid = PROC_SELF, .nd = &mnt_ns_desc };
 
 	if (!(root_ns_mask & CLONE_NEWNS))
 		return rst_collect_local_mntns();

proc_parse.c

@@ -944,8 +944,7 @@ struct mount_info *parse_mountinfo(pid_t pid, struct ns_id *nsid)
 	FILE *f;
 	char str[1024];
 
-	snprintf(str, sizeof(str), "/proc/%d/mountinfo", pid);
-	f = fopen(str, "r");
+	f = fopen_proc(pid, "mountinfo");
 	if (!f) {
 		pr_perror("Can't open %d mountinfo", pid);
 		return NULL;